1. Field of the Invention
The present invention generally relates to methods for joining and repairing articles formed from aluminum and aluminum alloys, such as the repair of heat exchangers used as engine radiators and air conditioning condensers. More particularly, this invention relates to a repair technique that entails the use of a soldering rod whose composition facilitates the repair of aluminum articles in an open atmosphere.
2. Description of the Prior Art
Heat exchangers are routinely employed within the automotive industry, such as in the form of radiators for cooling engine coolant, condensers and evaporators for use in air conditioning systems, and heaters. In order to efficiently maximize the amount of surface area available for transferring heat between the fluid within the heat exchanger and the environment, the design of the heat exchanger is typically of a tube-and-fin type containing a number of tubes which thermally communicate with fins. The fins enhance the ability of the heat exchanger to transfer heat from the fluid to the environment, or vice versa. Increasingly, heat exchangers used in the automotive industry are being formed from aluminum alloys so as to help reduce the weight of automobiles. Furthermore, the trend in the industry is to form aluminum alloy heat exchangers by a brazing operation, wherein the individual components of the heat exchanger are permanently joined together with a braze alloy such as Al-Si, whose solidus temperature is lower than that of the aluminum alloy being brazed. Brazing temperatures are generally considered to be about 569.degree. C. and above, in contradistinction to soldering which is generally performed at about 425.degree. C. and below.
One brazing technique which has become accepted by the automotive industry requires a furnace operation in an inert atmosphere. Prior to brazing, the surfaces of the components are coated with a flux mixture that, upon melting, cracks and displaces the aluminum oxide layer that naturally forms on aluminum and aluminum-containing alloys, such that the brazeability of the braze alloy is enhanced. Accordingly, flux compounds are selected so as to melt at a lower temperature than the braze alloy used, e.g., about 10.degree. C. to about 100.degree. C. lower than the solidus temperature of the braze alloy, such that the flux is able to displace the aluminum oxide layer prior to melting and flowing of the braze alloy. A conventional flux mixture consists of a flux compound suspended in water, with a widely used flux being potassium fluoroaluminate complexes, as disclosed in U.S. Pat. Nos. 3,951,328 and 3,971,501 to Wallace et al. and Cooke, respectively, as well as U.S. Pat. No. 5,242,669 to Flor and Conn assigned to the assignee of the present invention, and others. Other flux compounds finding use include cesium fluoroaluminate complexes, as disclosed in U.S. Pat. No. 5,360,158 to Conn et al., assigned to the assignee of the present invention, and U.S. Pat. Nos. 4,655,385 and 4,689,092 to Suzuki et al. Cesium fluoroaluminate complexes generally have lower melting temperatures (e.g., about 450.degree. C.) than potassium fluoroaluminate complexes (e.g., about 560.degree. C.), which allows for the use of braze alloys with lower solidus temperatures. After depositing the flux mixture, the assembly is dried to evaporate the water, leaving only the powdery flux solids on all of the external surface of the assembly. Brazing is then performed by heating in the inert furnace atmosphere, upon which the flux compound melts and displaces the aluminum oxide on the surfaces to be brazed, followed soon after by melting and flowing of the braze alloy onto the surfaces being joined.
A disadvantage with the conventional use of flux compounds suspended in water is the general inability to consistently deposit these flux mixtures on a limited region of the components being coated. In addition, after evaporation of the aqueous solvent, the flux has a particulate shape which does not adhere well to the surfaces of the heat exchanger. Subsequent handling and assembly of the heat exchanger causes sufficient agitation to shake loose a portion of the flux particles from the heat exchanger surface. Another shortcoming is that during brazing, it is extremely important that the furnace atmosphere have a dewpoint of about -40.degree. F. or below and a free oxygen level of about 100 parts per million or less in order to minimize oxidation of aluminum during the brazing cycle.
Methods for repairing aluminum components such as heat exchangers are also complicated by the above-noted circumstances. Particularly difficult are repairs that must be performed outside a brazing furnace, as is the case with most field repairs. While torch brazing is possible, particularly with cesium fluoroaluminate fluxes, the ability to provide an adequate amount of flux at the brazing site and the high temperatures required for brazing render field repairs very difficult. As a result, successful torch brazing outside an inert atmosphere furnace generally requires a specially formulated brazing paste such as that disclosed in U.S. Pat. No. 5,226,974 to Conn, assigned to the assignee of the present invention. The high temperatures of the brazing operation can also be detrimental to nearby regions or structures, such as the fins of a heat exchanger, which can be damaged or distorted during repairs performed at brazing temperatures.
From the above, it is apparent that it would be desirable to provide an improved method for repairing aluminum articles and structures, such as by simplifying how the flux and alloy are delivered to the site being repaired and by reducing the temperature necessary to perform the repair.